Just buy 5 or 6 2wd cars like I have and be done with it. One for everything, one for each hand, one for each track, one for each weather condition.
Personally I don't see an issue with running a different layout chassis for the conditions. It actually makes sense to have say a wet and dry car, high grip/low grip, or an indoor or outdoor car, etc.
Someone with more money available to spend will always find a way to throw more money at it if chassis are limited to one/meeting. Fresh tyres every run, for example.
Also with some chassis you are able to swap gearbox layouts, so one chassis rule still allows for significant change to the handling of the car for the prevailing conditions, which someone without that capability may claim that as unfair.
Don't let money or lack of it be an issue to your enjoyment of your racing, its not . Too many strict rules makes racing dull.